The invention relates to television systems and more particularly to the measurement of chroma key area in such systems.
In television newscasting, for example, it is known (see FIG. 1) to transmit a television picture 10 which includes an announcer 11 and a rectangular screen 12 onto which a scene 13 relevant to that part of the programme is superimposed.
The scene is not actually projected onto screen 12 but is electronically inserted by the system shown in FIG. 2 prior to transmisson. A first camera 20 receives the image of the announcer 11 and the blank screen 12. A second camera 21 receives the image of scene 13. The outputs from cameras 20,21 are received by an electronic switching unit 23 which is under the control of chroma key generator 24. Generator 24 also receives the output from camera 20. The screen 12 is normally a uniform blue colour and this is termed the chroma key (also known as CSO Chroma Separation Overlay). Selection of this blue colour allows the generator 24 to distinguish over other parts of the picture. The output from camera 20 normally passes to the output when switch 23 is in the position shown. When chroma key is present however switch 23 is changed under the control of generator 24 to the other position so that a portion of scene 13 is inserted during the presence of chroma key. Thus line by line the scene 13 is inserted into that portion of the picture containing chroma key. As shown part of the screen 12 is obscured by the announcer. This will prevent the chroma key for that part of the screen being detected so that the corresponding portion of the scene 13 will not be inserted thus generally avoiding partially obscuring the announcer.
Whilst the above known system is generally satisfactory, when the screen is not rectangular, for example due to partial obscuring by the announcer, the known detector system has difficulty in clearly determining the edge of the screen and thereby gives rise to the ragged or fuzzy edges commonly seen around the announcer's head. Thus such a detector system is unsuitable for use in accurately computing the key area size and position on its own. Another problem is that the system of FIG. 2 requires that cameras 20, 21 be in fixed positions because it can be seen that if camera 20 were zoomed in then screen 12 would be too large for scene 13 which would only partially fill the screen. Alternatively, if camera 20 were zoomed out then part of scene 13 would be lost. Sideways movement of camera 20 would give similar problems.
Co-pending patent application 42751/76 (U.S. patent application Ser. No. 841,519, now U.S. Pat. No. 4,163,249) describes a system (Quantel DPE 5000) able to modify the size and position of a T.V. picture. Therefore, if this system were to be given a measure of the size and position of screen 12, the DPE 5000 would be able to modify the size and position of the picture of scene 13 from camera 21 so that when keyed into the picture from camera 20, scene 13 fits the area of screen 12 exactly.